The spread spectrum communication (hereinbelow abbreviated to SSC) is a communication system, by which the band width of an information signal to be transmitted is spread several tens of times to several hundreds of times for transmitting the signal and as features thereof secrecy and concealing property can be cited. Further, since the electric power density is low, it is compatible with the narrow frequency band communication effected heretofore.
In the SSC as described above and in particular in an SS receiver, it is necessary to detect a carrier sense signal from a received signal and to obtain a signal for starting an initial synchronization circuit for a pseudo noise code therefrom. In a Japanese patent application entitled "Spread Spectrum Receiver" filed Oct. 9, 1987 by the applicant of the present invention (application Ser. No. , later published as JP-P-1-98338A, and corresponding to U.S. Pat. No. 4,943,975 synchronization circuit in the following stage is started by the carrier sense signal and a signal indicating the state of the receiver (reception wait state). This signal is described as a "receiving operation starting pulse" in the patent application stated above.
When the SS receiver detects the SS signal, it outputs the carrier signal. At this time, if the receiver is not in the state, where it can receive the signal, the initial synchronization circuit effects nothing. On the contrary, if the receiver is in the state, where it can receive the signal, and the reception wait signal is present, the initial synchronization circuit begins an operation of the initial synchronization for the pseudo noise code. When the initial synchronization is terminated, a starting pulse is outputted for performing the synchronization (frame synchronization) for detecting next the starting position of data. A frame synchronizing circuit is started by this signal to detect the starting position of data. When the frame synchronization is terminated, the wait state of the receiver is removed and demodulation is performed from the starting point of the data. The timing of this operation is indicated in FIG. 3.
By this carrier sense method the output of a correlator is counted and the count output is used as the carrier sense signal. Further, as a general carrier sense method, there is known a method, by which the AGC voltage of the high frequency amplifying section is used as the carrier sense signal, as disclosed e.g. in JP-P-2-123838A.
By this method, in an SS receiver having a correlator 30, a variable gain amplifier 31, a demodulator 32, an AGC circuit 33, etc., as indicated in FIG. 8, a gain control signal is outputted to the variable gain amplifier 31, responding to a correlation spike outputted by the demodulator and at the same time the carrier sense signal is obtained, responding to this gain control signal.
However, in the case where the output of the correlator is used as the carrier sense signal, an SSC signal having a different code cannot be detected and on the other hand, in the case where the AGC voltage of the high frequency amplifying section is used as the carrier sense signal, there is a drawback that the SSC signal cannot be detected correctly, because the initial synchronization circuit in the receiver reacts not only to the SSC signal but also to a signal of the narrow band communication.